Strapping made of metal or high strength plastic has long been used to secure the packaging of heavy boxes, pallets loaded with bricks and other heavy objects, large textile bales, and other packaging applications which require high strength reinforcement. Common materials used for the strapping include metal, polyester and polypropylene. Metal strapping is quite strong, but is also relatively expensive. Polypropylene strapping is less expensive, but may stretch longitudinally and loosen when under high tension. Polyester strapping is less expensive than metal strapping, is very strong, and is not easily stretched. For this reason, polyester strapping is useful in a wide variety of reinforcing applications.
Polyester strapping is commonly produced by forming continuous strips of polyester using a spinneret or other extrusion die, and molecularly orienting the strips in the longitudinal direction under conditions of heat and tension. The molecular orientation increases the strength of the strapping in the longitudinal direction. However, as the polyester molecules become more aligned in the longitudinal (machine) direction, they become less entangled in the lateral (transverse) direction. As a result, the increased strength of the strapping in the longitudinal direction resulting from the orientation, comes at the expense of reduced strength in the lateral direction. When the polyester strapping is pulled tight in the longitudinal direction during use, the resulting necking and bending stresses in the lateral direction may render the polyester strapping more susceptible to split in the longitudinal direction along a substantial length, ranging from a few centimeters to one meter or more.
Various attempts have been made to reduce the longitudinal splitting of polyester strapping by adding elastomeric ingredients or subjecting the strap to specialized processing, for instance, as described in U.S. Pat. No. 6,210,769. To date, these attempts have not resulted in a practical, cost effective technology to reduce split.